linux-kernel - Re: [PATCH] LoongArch: BPF: Sign-extend return values

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Message-ID: <6741fb6c516cc_c6be20839@john.notmuch>
Date: Sat, 23 Nov 2024 07:57:32 -0800
From: John Fastabend <john.fastabend@...il.com>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>, 
 John Fastabend <john.fastabend@...il.com>, 
 Huacai Chen <chenhuacai@...nel.org>
Cc: loongarch@...ts.linux.dev, 
 bpf@...r.kernel.org, 
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] LoongArch: BPF: Sign-extend return values

Tiezhu Yang wrote:
> On 11/22/2024 08:41 AM, John Fastabend wrote:
> > Tiezhu Yang wrote:
> >> (1) Description of Problem:
> >>
> >> When testing BPF JIT with the latest compiler toolchains on LoongArch,
> >> there exist some strange failed test cases, dmesg shows something like
> >> this:
> >>
> >>   # dmesg -t | grep FAIL | head -1
> >>   ... ret -3 != -3 (0xfffffffd != 0xfffffffd)FAIL ...
> 
> ...
> 
> >>
> >> (5) Final Solution:
> >>
> >> Keep a5 zero-extended, but explicitly sign-extend a0 (which is used
> >> outside BPF land). Because libbpf currently defines the return value
> >> of an ebpf program as a 32-bit unsigned integer, just use addi.w to
> >> extend bit 31 into bits 63 through 32 of a5 to a0. This is similar
> >> with commit 2f1b0d3d7331 ("riscv, bpf: Sign-extend return values").
> >>
> >> Fixes: 5dc615520c4d ("LoongArch: Add BPF JIT support")
> >> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> >> ---
> >>  arch/loongarch/net/bpf_jit.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
> >> index 7dbefd4ba210..dd350cba1252 100644
> >> --- a/arch/loongarch/net/bpf_jit.c
> >> +++ b/arch/loongarch/net/bpf_jit.c
> >> @@ -179,7 +179,7 @@ static void __build_epilogue(struct jit_ctx *ctx, bool is_tail_call)
> >>
> >>  	if (!is_tail_call) {
> >>  		/* Set return value */
> >> -		move_reg(ctx, LOONGARCH_GPR_A0, regmap[BPF_REG_0]);
> >> +		emit_insn(ctx, addiw, LOONGARCH_GPR_A0, regmap[BPF_REG_0], 0);
> >
> > Not overly familiar with this JIT but just to check this wont be used
> > for BPF 2 BPF calls correct?
> 
> I am not sure I understand your comment correctly, but with and without
> this patch, the LoongArch JIT uses a5 as a dedicated register for BPF
> return values, a5 is kept as zero-extended for bpf2bpf, just make a0
> (which is used outside BPF land) as sign-extend, all of the test cases
> in test_bpf.ko passed with "echo 1 > /proc/sys/net/core/bpf_jit_enable".
> 
> Thanks,
> Tiezhu
> 

Got it.

Acked-by: John Fastabend <john.fastabend@...il.com>